1. Field of the Invention
The invention relates to a device for monitoring the stack exit air in a reactor installation, essentially consisting of a pressure relief line which connects the containment vessel to the stack and in which a filter unit is located, a sampling point being provided downstream of the filter, from where gas mixture is branched off via a sampling line, passed through a measurement section and then returned to the stack exit air.
2. Discussion of Background
The atmosphere in the containment vessel of a nuclear power station consists as a rule of air, steam, hydrogen, CO.sub.2, rare gases, iodine and aerosols. In normal operation of the installation, this mixture, which has an activity of about 10.sup.3 Bq/m.sup.3, is discharged from the containment vessel via a venting unit directly into the stack. In the event of an accident with a small leakage in the primary system, during which the activity is between 10.sup.3 Bq/m.sup.3 and 10.sup.8 Bq/m.sup.3, the gas is likewise discharged via the venting unit directly into the stack. In the event of a major accident with, for example, core meltdown, the activity can become greater than 10.sup.14 Bq/m.sup.3. During a major accident, the venting unit is isolated, whereupon the pressure in the containment vessel rises. To avoid an unduly great pressure rise, the containment vessel is relieved via a filter unit. In this filter unit (for example a dry filter or wet filter), the activity of iodine and aerosols is reduced by a factor of at least 1,000. Downstream of the filter unit, the activity of the gas is then determined in a measurement section.
Because of the remaining very high activity in the downstream pure-gas line leading to the stack, the measuring apparatus used for normal operation, such as balancing filters and aerosol monitors cannot be used, since the measuring range would be exceeded and handling of the balancing filter would not be ensured. For this reason, special instruments having a wider measuring range and involved screenings as well as complicated devices for handling the balancing filter are normally used.